In a lithographic duplicating process, an image to be reproduced is placed on a lithographic master and the master is attached to a master cylinder of the duplicating machine. The image on the master attracts the grease-base ink and repels water, while the non-image or background portions of the master attract water and repel ink.
With each revolution of the master cylinder, the film of water or dampening fluid and the film of ink are replenished on the surface of the master on the cylinder. The water allows the ink film to adhere only to the image areas of the master that are to be reproduced. Thereafter, the film of ink is transferred from the image on the master to a blanket on a blanket cylinder and from there to a copy sheet as it passes between the blanket and impression cylinders.
Because of variable factors such as humidity, temperature, the type of master and copy paper being used, etc., all of which have an influence on the quality of the copy reproduced and to a great extent determine the amount of water or dampening fluid that should be supplied to the master, control and metering of the water is critical in the production of high quality duplicated copies.
There are many known devices for controlling the rate feed of dampening fluid in offset duplicating machines, and it is customarily found convenient to effect this control by means of an adjustable drive for incremental rotation of the fountain roller by selected increments. The fountain roller conveys the fluid to a roller of a moisture roller train which transfers the fluid via other rollers in the train to the master on the master cylinder of the machine.
While such devices have been fairly successful, they are somewhat limited in being able to control the rate of feed of the dampening fluid in extremely precise and accurately metered quantities. Further, operation of these devices require a certain amount of skill in order to obtain the proper moisture balance for the particular requirements of the duplicating process. Thus, devices of this kind are not simple and easy to use by clerical personnel operating office-type duplicating machines, who are normally unskilled in the printing trade.
One of the steps normally required to assure adequate quality of the copies produced in this process is to properly time the turning on and turning off of the moisture system in accordance with the feeding of sheets through the duplicator. For example, it is essential to turn on the moisture system early enough so that the first sheet of a new run receives an adequate film of moisture. Providing the appropriate automatic controls to accomplish this is accordingly an important purpose of this invention.
A moisture control apparatus providing for infinitely variable control and adjustment for precisely metering the rate of feed of the dampening fluid supplied to the master is disclosed in copending application Ser. No. 678,219, filed Apr. 19, 1976, assigned to the assignee of the present invention.
As disclosed therein, the apparatus comprises linkage means for converting continuous rotary input motion to incremental rotation of the fountain roller. The apparatus includes a drive link for imparting motion to the fountain roller and a control link for controlling the magnitude of incremental rotation, the drive and the control links being supported on independent pivots but both being swingable about a common adjustably positionable fixed pivot. The frequency of the incremental rotation of the fountain roller is proportional to the rotational speed of the duplicator, and the magnitude of the individual roller incremental motions is substantially proportional to an angle formed between the drive and the control links as determined by the position of the fixed pivot which is selectively settable to an infinitely variable position.
The foregoing apparatus provides a highly successful arrangement for controlling the rate of feed of the dampening fluid in extremely precise and accurately metered quantities. It also provides a device which is simple and easy to use by machine operators who are relatively unskilled in the printing trade.